1. Field of the Invention
This invention relates to an apparatus for covering closely at least one surface of a base sheet such as card, tag or label with a plastic film. More particularly, it relates to an apparatus of the above type, which is provided with heating and pressing rollers and a heat radiation lamp as a heating source for the rollers, whereby preheating the base sheet and the plastic film overlaid by a part of the heat radiated from the heat radiation lamp and thus featuring a rather short waiting period prior to the initiation of the covering work.
2. Description of the Prior Art
For a base sheet such as "name card," "identification card," "commutation ticket," "miniature calendar," or other card, tag or label, it is generally advantageous and, in many instances, is required to provide a surface covering which may be considered to be one mode of packaging so as to impart durability and water proofness thereto or to improved the durability and water proofness thereof and/or to prevent any unauthorized alteration of information shown on the base sheet.
To meet such a demand as mentioned above, a variety of base sheet surface covering apparatus have heretofore been proposed.
FIG. 1 is a fragmentary schematic illustration of an example of such prior art base sheet surface covering apparatus. It is designed to cover both surfaces of a base sheet. A base sheet travelling path P consists of a base sheet feed path P1, a base sheet heating and press-bonding path P2 and a base sheet discharge path P3, and a base sheet 2 to be covered on which a covering plastic film 1 is superposed in such a manner that the base sheet is interposed into a folded-shaped covering plastic film for example is conveyed and travels along the base sheet travelling path P. A pair of hollow rollers 3A, 3B are arranged on the base sheet heating and press-bonding path P2 in such a way that they abut on one another, thereby constituting a pinch roller unit 3. In addition, heaters 5A, 5B are disposed respectively in the internal spacings 4A, 4B of the rollers 3A, 3B.
An example of the above-mentioned covering plastic film 1 is shown in FIG. 2. On a surface of a surface layer CL made of a polyester film or the like, there is formed a bonding layer AL consisting of a thermoplastic resin such as polyethylene or the like so as to make up the covering plastic film. In the illustrated example, two film pieces are put together at their edges E with their bonding layers AL facing one another.
In a base sheet surface covering apparatus of such a construction as described above, the rollers 3A, 3B are heated from their interior by means of their respective heaters 5A, 5B. A base sheet 2 with the plastic film 1 superposed thereon is conveyed along the base sheet travelling path P from the base sheet feed path P1 to the base sheet heating and press-bonding path P2, where the plastic film 1 is heated and pressed by the rollers 3A, 3B and fused to stick with the base sheet 2, thereby completely covering the base sheet 2 at both surfaces thereof. Then, the resulting base sheet 2 is discharged from the apparatus along the base sheet discharge path P3.
Needless to say, one of the heaters 5A, 5B may be omitted if the above apparatus is adapted to cover one surface only.
In such a conventional base sheet surface covering apparatus as described above, the wall thickness of each of the rollers 3A, 3B is generally made relatively thick with a view toward heating uniformly the film 1 and the base sheet 2 to be covered. As a corollary to this, the rollers 3A, 3B have a large heat capacity. To use the apparatus when the rollers 3A, 3B have been cooled down due to, for example, a relatively long work interruption period over a night or the like, it takes a long time period (for example more than 5 minutes) after the starting of energizing of the heaters 5A, 5B until their temperatures reach a sufficient level for achieving a desired coverage, resulting in a drawback that a long waiting period is indispensable.
It may be effective to heat the circumferences of the rollers 3A, 3B by radiant heat, for example, in order to solve the above drawback. It may be feasible to shorten the waiting period by applying such radiant heat to the circumferences of the rollers 3A, 3B to raise the temperatures of the circumferences of the rollers 3A, 3B relatively fast. However, when the rollers have not yet been warmed up in their entirety, for example, immediately after turning on the heaters to use the covering apparatus, the radiant heat is absorbed in the interiors of the rollers and the film 1 and the base sheet 2 may not be applied with enough heat generally required at the base sheet heating and press-bonding path P2. Furthermore, in order to carry out the surface covering of the base sheet 2 at a high speed, it is certainly necessary to increase the temperatures of the rollers 3A, 3B still further. In this case, the base sheet 2 superposed with the plastic film 1 is abruptly subjected to a large temperature change and the base sheet 2, after the surface covering processing, may develop a deformation such as warping or twisting.